244 ESSAY 



Agreeably to another opinion, the dew found 

 upon growing vegetables is the condensed va- 

 pour of the very plants, on which it appears. 

 But this also seems to me erroneous for several 

 reasons. 1. Dew forms as copiously upon dead 

 as upon living vegetable substances, 2. The 

 transpired humour of plants will be carried 

 away by the air which passes over them, when 

 they are not sufficiently cold to condense the 

 watery vapour contained in it ; unless, which is 

 almost never the case if mist does not already 

 exist, the general mass of the atmosphere be 

 incapable of receiving moisture in a pellucid 

 form. Accordingly, on cloudy nights, when 

 the air, consequently, can never be cooled more 

 than a little below the point of repletion with 



sunrise, and the ceasing of the formation of dew upon grass 

 in the morning. These observations were made on spots 

 exposed during the greater part of the day to the sun. In 

 such places, the heat acquired, from the sun, by the upper- 

 most layer of earth, will be longer retained, than that ac- 

 quired by the grass, which will, therefore, be sufficiently 

 cool, soon after the heat of the day has declined, to condense 

 a part of the vapour then copiously rising from the earth; 

 whereas in the morning, both less vapour will rise, the sur- 

 face of the earth having now lost a great part of its heat, and 

 a less proportion of that which does rise will be condensed 

 by the grass, as the temperature of this body now more 

 nearly approaches that of the ground, from first receiving 

 the heat of the sun reflected from the atmosphere and other 

 substances. 



